The present invention relates to a double impeller for stirring particularly sterile liquids. More particularly, it relates to an impeller which has an impeller head with a stirring tool and a lower opening for receiving a pin in a central hollow chamber, wherein the impeller is driveable inductively or magnetically in a contactless manner.
Such an impeller is disclosed for example in U.S. Pat. No. 928,841 and in the European patent document EP-A10399972. The impeller has a plurality of radially outwardly extending stirring blades forming a stirring tool. When the impeller is arranged in a container a flow is produced which forms in the container a liquid whirl flow with the impeller as a center point. A negative pressure is formed above the impeller and under the action of suction, the upper surface of the liquid lowers conically in direction of the impeller. The negative pressure acting in this region is not desirable since it contributes to the entrapment of the gasses into the liquid and in some liquids can form foams.